Ever wonder how much an exploit for a previously unknown vulnerability that affects all Windows versions costs on the black market? The answer, according to a recent offer seen on a cybercrime forum, is $90,000.

The offer was observed by researchers from security firm Trustwave on an underground market for Russian-speaking cybercriminals, where users hire malware coders, lease exploit kits, buy access to compromised websites or rent botnets.

Zero-day exploits -- exploits for unpatched vulnerabilities -- are typically used for cyberespionage. Hackers sell them to governments and large corporations, under strict non-disclosure agreements, often through specialized brokers, so it's uncommon to see them traded on cybercrime forums.

While it's hard to prove the authenticity of the offer without actually buying the exploit, there are strong indications that the author's claims are real, the Trustwave researchers said in a blog post.

The author went to great effort to prove that he has what he claims: a local privilege escalation exploit that works on all Windows versions since XP, including Windows Server editions, and bypasses common exploit mitigations like DEP, SMEP and ASLR.

The vulnerability is supposedly located in the win32k.sys kernel driver, which historically has been a source of many privilege escalation flaws. The exploit relies solely on the KERNEL32 and USER32 Windows libraries (DLLs), the seller claims.

The original starting price was $95,000, but it has since dropped to $90,000. For this sum, the exploit author offers the exploit's source code as well as consultation and help integrating it into the buyer's project.

While privilege escalation flaws do not, by themselves, allow the remote compromise of a computer system, they're still an important part of most attack chains. Many applications now run with limited privileges on Windows or have sandboxing mechanisms meant to prevent a full system compromise if an attacker finds and exploits a remote code execution vulnerability in them.

In such environments, attackers need privilege escalation exploits to gain system-level access and take full control of a computer, making them highly valuable. With system privileges attackers can then install rootkits and hide their malicious code from security products for increased stealth and persistence.

In a Windows server environment, a simple SQL or file injection vulnerability in a website can turn into a complete server compromise through such an exploit.

Based on the little data that has leaked into the public domain about exploit prices, $90,000 for a Windows privilege escalation exploit is "on the high end but still within a realistic price range, especially considering the return on investment criminals are likely to make using this exploit in any campaign," the Trustwave researchers said.

Microsoft defended its security measures. "Windows is the only platform with a customer commitment to investigate reported security issues, and proactively update impacted devices as soon as possible," a company representative said.

Microsoft recommends customers use Windows 10 and the Edge browser for the best protection, and the company's standard policy to update software during its regular Update Tuesday schedule, the company said.

If the exploit seller's claims are accurate, it's hard to defend against this exploit, especially since a demo video shows the exploit successfully bypassing all the protections enforced by Microsoft's Enhanced Mitigation Experience Toolkit (EMET).

That said, users should make sure that they take the common precautions like keeping their software up-to-date and running a capable security product. This could break a different link in a potential attack chain, such as a remote code execution exploit needed to gain access to the system in the first place.